Linda Chapman 
Unicorn School: The Surprise Party 

In this second adventure, Troy has been behaving very strangely. The others find out that he is really a unicorn prince. Now that his secret is out, Troy thinks they only want to be his friend because he is royal. Willow tries to reassure him that he is still the same old Troy to them and sets out to prove that being a loyal friend is more important than where you come from.